We have a local cacher who has placed over a hundred caches but hasn't been active for more than a year. Although some of his caches are getting archived through a long period of "DNFs" then "needs maintenance" then "needs archived" and the reviewer waits a month for a response from the CO (which isn't forthcoming) before it's officially archived, many of his caches are still being found. So archiving them isn't really necessary. And I don't know what the rules are, but I don't think a reviewer can archive a cache if it's still being found if the CO hasn't given permission. I think the other issue with this plan is that a reviewer would have to keep track of the logins of all the cachers in his ares (which could be thousands) to see if they are still active. A daunting task to say the least.
That's a good idea but unfortunately, a CO has to place a cache up for adoption and/or invite a specific person to adopt the cache before it can happen. We attempted to adopt a cache that had had many DNFs but was in a nice location. We wrote to the CO and asked if we could adopt their cache but they hadn't been active in a long time and they never wrote back to us. Eventually we added a "needs archived" note and after some time the reviewer archived it freeing up the location for a new cache.
What should be done is instead of archiving it first the reviewer could post a note saying this cache is up for adoption and after a time if there are no hits then its deleted.